The primary purpose of the research described is to determine whether the Octodon degus, a South American, diurnal, hystricimorph rodent is a suitable animal model of diurnal circadian rhythms to eventually use in studies of human circadian disorders and research related to the general neurobiological control of circadian rhythms in diurnal mammals. Previous attempts to develop a diurnal animal model have been limited by difficulty obtaining sufficient animals, circadian rhythms were sufficiently noisy so as to exclude studies of the formal properties of the circadian system, behaviors in the laboratory did not agree with those reported in the field, or methodologies were limited for gathering and analyzing data. To date, few researchers have attempted to find a suitable day-active species for extensive description of the formal circadian properties. Development of a diurnal rodent model will allow a more cost-effective means than using human subjects or primates to develop treatments for human disorders such as the problems that result from transmeridian jet travel, shiftwork, illness and hospitalization for traumatic problems. These situations cause deterioration of mental alertness, sleep disruption, ulcers and, in the most severe cases, depression. A good diurnal rodent model will also allow extensive comparative physiological and neurobiological studies. Thus, the main objective of this project is the determination of the formal circadian properties of the degu to determine whether it may prove to be a model diurnal species. The data currently available for diurnal species indicate that circadian function in such species may differ substantially from that of nocturnal species. To determine the formal circadian properties, the following parameters will be examined in adult male and female degus: 1) The stability and precision of entrained circadian activity rhythms. 2) The reentrainment response (time and form) to various size phase-shifts in the light-dark (LD) cycle. 3) Stability of free-running rhythms and their dependence on constant levels of illumination (Aschoff's Rule). 4) Development of phase response curves (PRC's) using 1 hr light pulses for animals free- running in constant darkness (DD). 5) The limits of entrainment (various LD ratios). 6) Relative importance of LD amplitude in entrainment. 7) Comparison of activity and temperature rhythm entrainment and free- running rhythms. 8) Ability of social cues to act as a non-photic entraining agent.